I have an idea for a model and could easily arrange a couple of coaxial propellers. I was thinking a 5x3 in front of a 5x5 on a single shaft, or 2 motor counter rotating. Would this multi stage approach give good thrust characteristics for a smallish prop length?

Looks interesting, I was going to point one motor forwards and one back. I was told that using too high a pitch too fast just has a mixing effect on the air instead of thrust. So I thought that having a lower pitch prop in front could speed up the air a bit before it hits the high pitch prop.

You mean more like the Cessna Skymaster, or Dornier Do 335, with an engine at the front and one at the rear ?

There is also the Dornier Do 18, and the more up to date Dornier Seamaster with a push-pull twin engine arrangement in a single nacelle.

Whatever type you decide on, from a model point of view I would recommend searching for what other modelers have used in a similar cofiguration, and how successful it may have been. The eventual proof will most likely be how the model eventually flies.

Sorry I can't recommend which way to go regarding props and pitches, but I'm fairly sure someone will have already done what you are thinking of. Always easier to pick someone else's brain, (design), that has had success.

And...
These done to test rudder authority. The rudder ran out of authority at about 40% power on the running motor.
The inline push-pull thingy only resulted in amusing videos of the props running. The airplane was a miserable failure.